If the swans do not migrate, they will try to find a body of water unfrozen so that they can stay warm and sleep at night protected from predators.

Swan feathers are made for cold weather, even tundra-like conditions. However, the swans' feet could easily freeze on ice, the swans could drown, starve, become dehydrated, killed by predators which can access the ice and any other number of problems associated with frozen water. So, their best defense is to migrate from such inclement weather.

If the swans can find a body of water that is protected and does not freeze, their feet will stay warm in the water and they will be protected from predators. The Regal Swan